A lot of attention has been paid to the technology of forming a field-effect transistor, for example a thin film transistor (TFT), by using a semiconductor thin film that is formed over a substrate having an insulating surface. The thin film transistor is used for a display device typified by a liquid crystal television. Although silicon-based semiconductor material is known as a material for a semiconductor thin film which can be applied to the thin film transistor, an oxide semiconductor has attracted attention as another material.
As the material for the oxide semiconductor, zinc oxide and a material containing zinc oxide are known. A thin film transistor formed using an amorphous oxide (oxide semiconductor) whose electronic carrier concentration is lower than 1018/cm3 is disclosed (Patent Documents 1 to 3).